1. Field of the Invention:
This invention relates to a focus detecting device arranged to divide the pupil of an objective lens into a plurality of regions and to detect the focused state of the objective lens through the relative positions of a plurality of images formed by light fluxes passing through these regions and more particularly to an optical system to be employed in the focus detecting device.
2. Description of the Related Art:
Varied devices of the above stated kind have been proposed for use in cameras or the like. FIG. 1 of the accompanying drawings shows an example of the arrangement of optical systems being used for the conventional devices. An improvement on this arrangement has been disclosed in U.S. Pat. No. 4,699,493.
Referring to FIG. 1, the illustration includes an objective lens 1; a predetermined image forming plane 2 on which an image is to be formed by the lens 1; a field mask 3; a field lens 4 disposed close to the predetermined image forming plane 2; secondary image forming lenses 5a and 5b which have their optical axis in parallel to each other and are symmetric with respect to the optical axis of the objective lens 1; aperature stops 6a and 6b to which are arranged for the secondary image forming lenses 5a and 5b respectively; sensor arrays 7a and 7b which are disposed respectively behind and for the secondary image forming lenses 5a and 5b; and exit pupil regions 8a and 8b of the objective lens 1. Each of the secondary image forming lenses 5a and 5b is a positive lens. Further, the field lens 4 is arranged to form images of the aperture stops 6a and 6b in the vicinity of the exit pupil regions 8a and 8b of the objective lens 1. Light fluxes passing through these regions 8a and 8b are arranged to be incident on the sensor arrays 7a and 7b respectively. The light, is not limited to visible rays of light but invisible rays of light such as infrared rays also may be used.
A focus detecting device having the optical system which is arranged as described above is arranged as follows: In case the image forming point of the objective lens 1 is in front of the predetermined image forming plane 2, the secondary images which are formed on the two sensor arrays 7a and 7b are close to each other (toward the optical axis). When the image forming point of the lens 1 is in the rear of the predetermined image forming plane 2, the secondary images formed on these sensor arrays 7a and 7b are away from each other. The amount of deviation from each other of the two secondary images formed on the sensor arrays 7a and 7b is in a certain relation to the degree of defocus of the objective lens 1. The degree and the direction of the defocus of the objective lens is, therefore, detectable by computing the amount of deviation with some suitable computing means. One of known methods for computing the amount of deviation from each other of the two secondary images is disclosed, for example, in Japenese Patent Application Laid-Open No. SHO 59-107313 (corresponding to U.S. Pat. No. 4,559,446). This method is as briefly described below:
The outputs of the sensor arrays 7a and 7b are photo-electric converted. Assuming that the converted outputs are a(i) and b(i), wherein i is the number of from 1 to N, with N assumed to be the number of picture elements of each of the sensor arrays, and with a suitable constant integer k used, computation is performed for different integers m in accordance with the following formula: ##EQU1## The range of i for obtaining a sum is set at such a value as to have i, i+k-m, i+k, i-m within a closed interval [1, N] and to have the variation ranges of the first and second terms of V(m) equal to each other. With one pitch of each of the sensor arrays used as a unit, a value of m which makes the value of V(m) zero represents the amount of deviation of the secondary images from each other. Generally, it is not always possible to have the integer m at such a value that makes the value of V(m) zero. This problem is soluble by obtaining a fraction in accordance with some suitable interpolating method. The simplest of interpolating methods are linear interpolation. Assuming that the sign is inverted between a value V(mO) and a value V(m1), the amount M of deviation which includes a fraction can be obtained from the following formula: EQU M=m0+.vertline.V(m0).vertline./ (.vertline.V(m0)+V(m1).vertline.)(2)
With the amount M of deviation from each other of the two secondary images thus obtained, the simplest method for computing the defocus degree d of the objective lens 1 is as follows: Assuming that the two images are approximately in proportion to each other, the defocus degree d can be obtained by using a constant of proportion k in accordance with the following formula: EQU d=k(M-.delta.0) (3)
wherein .delta.0 represents an amount of deviation from each other of the two secondary images obtaine when the objective lens 1 is in an in-focus state. Hereinafter this value .delta.0 will be referred to as an initial deviation value. The initial deviation value .delta.0 is determined either during designing work on the device or by performing mesaurement during adjustment work in the initial stage of the device. A computing method which is an improvement over Formula (3) and gives the defocus degree d of the objective lens 1 with an improved degree of accuracy by performing a computing operation with the non-linear nature of the values d and M taken into consideration has been disclosed in Japanese Patent Application Laid-Open No. SHO 59-107313. However, since this method is not directly related to this invention, the details of that method are omitted herein.
As apparent from Formula (3) above, the focus detecting device uses the initial deviation degree .delta.0 as a datum. For accurate focus detection, this value .delta.0 must be unvarying. However, even if the initial deviation degree is at the value .delta.0, the various component members of the optical system vary as the circumstances vary. Therefore, the value .delta.0 does not remain unchanged. For example, a distance between the secondary image forming lenses 5a and 5b of FIG. 1 varies with changes in humidity and temperature. In that event, the value .delta.0 also changes. Generally, such variations due to temperature and humidity are not considered extreme. However, the degree of image deviation relative to the defocus degree d of the objective lens 1 is very small ranging from one tenth to several tenths of the defocus degree. Therefore, even a slight change in the value .delta.0 is detrimental to accurate detection of the focused state of the objective lens 1.
In case the optical members are made of an acrylic resin material in particular, they greatly expand and contract due to changes in temperature and humidity. In addition to this, their refractive index also changes to further affect the detection accuracy.
In one of a member of conceivable methods for solving this problem, a temperature and humidity sensor is provided within the optical system and the result of detection is corrected according to the output of that sensor. However, at present, it is hardly possible to find any sensor that is accurate and easy to handle. Even if such a sensor is obtainable, it would necessitate and additional space for that sensor and thus would result in an increase in cost. The adverse effect of temperature and humidity which varies every moment gradually permeates each component member from the surface thereof. Therefore, it is hardly possible to accurately make correction solely on the basis of the temperature and humidity obtained at the moment at which the correction is effected.